You are Here: Home >< Maths

# FP3 cross product proof of the direction Watch

1. I've seen a proof of 2 vectors a,b laxbl = lallblsin(angle between them)

https://math.la.asu.edu/~surgent/mat..._mag_proof.pdf

but I can't find a proof on google of why the direction of the vector from the cross product of a and b is perpendicular to both of them?

anyone got any good sites or videos, or can actually post their own proof? thanks
2. (Original post by physics4ever)
I've seen a proof of 2 vectors a,b laxbl = lallblsin(angle between them)

https://math.la.asu.edu/~surgent/mat..._mag_proof.pdf

but I can't find a proof on google of why the direction of the vector from the cross product of a and b is perpendicular to both of them?

anyone got any good sites or videos, or can actually post their own proof? thanks
There's no proof. It's simply the part of the definition of the cross product. It's defined that way essentially because there are physical vector quantities that combine in such as way as to produce a vector that is most usefully shown as perpendicular to the plane of the other two e.g.

a) the Lorentz force F on a moving charged particle in a magnetic field B is perpendicular to the plane containing B and the velocity vector v of the particle.

b) if we draw a torque vector as perpendicular to the plane containing the force and displacement vectors, then we find that the torque vectors add together correctly when we have multiple forces turning an object.

So it's defined to point in a perpendicular direction since that happens to model some physical situations nicely.
3. (Original post by atsruser)
There's no proof. It's simply the part of the definition of the cross product. It's defined that way essentially because there are physical vector quantities that combine in such as way as to produce a vector that is most usefully shown as perpendicular to the plane of the other two e.g.

a) the Lorentz force F on a moving charged particle in a magnetic field B is perpendicular to the plane containing B and the velocity vector v of the particle.

b) if we draw a torque vector as perpendicular to the plane containing the force and displacement vectors, then we find that the torque vectors add together correctly when we have multiple forces turning an object.

So it's defined to point in a perpendicular direction since that happens to model some physical situations nicely.
oh yeah haha thanks! lol, they should have just put that in my textbook... rather than just giving me an equation with no explanation

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 16, 2014
Today on TSR

### Getting into A* study habits

Your best bet is to get started right now...

### My girlfriend should pay half towards my car!

Discussions on TSR

• Latest
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams